Format: ORAL

Spencer C.H. Barrett

Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, Canada

Among the most remarkable floral strategies in flowering plants are those involving stylar polymorphisms. Unlike the majority of angiosperms with continuous phenotypic variation in style and stamen deployment, species with stylar polymorphism have discrete patterns of sex-organ variation and populations with morphologically identifiable, genetically controlled floral morphs. The most well-known stylar polymorphisms are distyly and tristyly. Since Darwins classic book on heterostyly Forms of Flowers ecologists and evolutionary biologists have been fascinated by the origins and adaptive significance of these polymorphisms. We now know that at least some of these polymorphisms are controlled by linkage groups composed of major genes, and that they are maintained in populations by negative frequency-dependent selection resulting from disassortative mating between morphs. Work on unrelated distylous and tristylous species has revealed much about the variation in expression and the functions of the various components of the heterostylous syndrome. However, our understanding of the other four classes of stylar polymorphism stigma-height dimorphism, enantiostyly, flexistyly and inversostyly is far less complete. Here, I summarize what we currently know about the six stylar polymorphisms in an effort to synthesize similarities and differences between the polymorphisms and general principles that may apply to all of them. I focus on recent work on three main topics: 1) the genetic architecture of distyly and tristyly supergenes and patterns of molecular convergence among unrelated lineages; 2) efforts to understand the molecular underpinnings of enantiostyly and flexistyly; 3) the ecology and evolution of breakdown in the maintenance of stylar polymorphisms and the genomic consequences of the resulting transitions in the mating biology of populations. I conclude by highlighting unresolved questions in the study of stylar polymorphisms and why these polymorphisms remain attractive systems for the study of adaptation and evolution.